1. Field of the Invention
The present invention relates to the method of and apparatus for the custom fitting of heat shrinkable film to compound curvatures, specifically on automotive glass.
2. Discussion of Prior Art
In the past, window film installation on the interior surface of automotive glass having compound curves required the use of multiple strips of film. The strips varied in width depending upon the severity of the compound curve and the installer's capabilities. Attempting a one-piece rear glass or side quarter installation was the equivalent of trying to gift-wrap a basketball without wrinkling the paper. It just could not be done.
Various techniques were used including overlapping strips, straight or curved "butt-seams" (achieved by overlapping strips of film, cutting through both layers of the overlap, and then removing the severed strips), and apparently "seamless" installations in which the defroster elements were used as a cutting guide for relief cuts, thereby camouflaging the actual seams. More recently, such film installations have used a heat shrinkable film for heat forming a window film. In such applications, the film is first heat-formed on the exterior surface of the glass so as to smoothly lay on the compound curvature to be subsequently adhesively applied to the inside surface.
FIG. 1A illustrates a compound curved glass window 10, perhaps in an automotive application, to which film 12 is to be applied. FIG. 1B shows the window on which, due to the compound curvature of the glass and the essentially planar nature of the film 12, a number of scallop-shaped "fingers" or darts of film 14 have been formed. Where the film 12 is a heat shrinkable film, it can be shrunk in what is known as the "machine direction" (MD). The machine direction is the direction that the film is pulled through processing rollers during the manufacturing process causing a certain amount of stretch to be imparted to the film in the MD. Little or no stretch is provided to the film in the "transverse direction" (TD), i.e. the direction traversing the processing roller width, i.e. normally right angles to the MD. When the film is heated as during the heat-forming process, the film will tend to return to its original length upon cooling, thus shrinking depending upon the amount of heat applied, up to 4 or 5%, allowing the film to be shaped to a compound curved surface.
Thus, it can be seen that the fingers 14 formed by the planar film laying on the compound curved glass surface are shrunk by the application of hot air from a conventional heat gun 16. As one might expect, the output of such a heat gun is a stream of hot air that has a temperature well above the melting point of the plastic film. As a result, it is necessary for the operator to constantly move the heat gun around and over the surface of the plastic film, at controlled distances, so as to heat it up enough to shrink the film, but not get it so hot that it begins to melt, burn, or otherwise excessively deform.
Conventional application techniques involve a rapid circular motion combined with a transverse pattern of movement in the machine direction. This movement pattern is shown in FIG. 1B as the dotted line path where each of the dots may represent a smaller circular motion with subsequent movement in the transverse or machine direction to another point and a similar circular motion. The result of this movement is a gradual overall shrinkage of the film with a greater degree of shrinkage at the lower edge than in the middle of the film. As can be seen, the complex pattern of rotating and transverse movement of the heat gun is necessary to avoid undue shrinkage in any one spot caused by an inordinately high heat rate applied to that portion of the plastic film. As would be expected, a substantial period of time is required to learn the technique and the learning is mostly by trial and error, resulting in substantial film wastage.
There has been a long-felt need for a system to permit a more controlled application of heat to of heat-shrinkable films in order to facilitate their application to compound curves found in many automotive windows. While the use of heat-shrinkable film and the complex technique of shrinking that film to fit a compound curvature is relatively well known, the technique requires a substantial learning curve with the result that a substantial portion of film is lost or damaged during the various trial fittings until the technique is learned.